This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Ribonucleotide Reductases (RNRs) play an essential role in DNA synthesis by reducing the hydroxyl group of all four types of ribonucleotide, producing the corresponding deoxyribonucleotides. Three separate classes of reductases are known, but in our study we emphasize on the Class I, which are O2 dependent. All class I RNRs are composed of two proteins R1 and R2 [unreadable]each of them is a homidimer by itself. R1 protein has the substrate binding site, whereas both polypeptide chains of R2 have a stable free radical on a tyrosine residue, neighboring an antiferromagnetically coupled high spin iron(III) [unreadable]iron(III) site. Recent studies on E. coli RNR showed successful substitution of iron cluster with manganese (II) and this provided a good opportunity for study of the metal cluster by EPR spectroscopy because manganese (II) - monomeric or dimeric is EPR active. Herein we performed a broad range of spectroscopic as well as activity assay investigation on the properties of Mtb RNR R2 subunit to bind iron or manganese. These properties were correlated to the ability for tyrosil radical formations and enzymatic activity. We observed a dimeric manganese (II) incorporation in the protein either in a growth medium with high manganese concentration or during reconstitution of "apo"R2 with oxygen free solution of manganese (II) ions.